1. Field of the Invention
The present invention relates to wireless communications. More specifically, the present invention relates to a system which enables a secondary network that uses OFDMA-based femtocell communication to coexist with a macro cellular network using the same spectrum.
2. Discussion of the Related Art
Mobile telephones are essential components that enable today's ubiquitous communication capability. Mobile telephones, which operate inside the coverage areas of service providers or operators, are becoming alternatives to fixed or “land-line” telephones. Recent trends of mobile phone usage are discussed, for example, in the article, entitled “UMA and Femtocells: Making FMC Happen” (“Choudhury”), by Partho Choudhury and Deepak Dahuja, published on-line as a White Paper, December 2007. Choudhury discloses that (a) approximately 30-35% of all voice calls are made over a mobile network are initiated by the subscribers at their homes, and (b) in 2006, about 35% of video streaming and broadcasting services over cellular wireless networks are received while the mobile users are at their homes.
The mobile telephones are becoming an individual's only telephone. Furthermore, mobile users that are under the age of 24 make up to 80% of their long distance calls on wireless networks, rather than wired networks. This statistic is reported in “Femto Cells: Personal Base Stations” (“Airvana”), Airvana Inc., White Paper, 2007. However, the reliability, voice quality and cost of today's mobile telephone networks in indoor environments are not at a desirable level. Typically, indoor mobile telephone service is costly, with many dead spots and poor coverage, resulting in poor customer experience, so that the mobile telephone cannot truly be the only telephone the subscribers need.
Recently, a new class of base stations designed for indoor and personal use is described in both Choudhury and Airvana above, and in “The Case for Home Base Stations” (“PicoChip”) PicoChip Designs Ltd., White Paper, April 2007. The cells using these indoor, personal base stations are referred to as “femtocells”, and they enable indoor connectivity through existing broadband Internet connections. Femtocells are also considered as one of the options for fixed-mobile convergence (FMC), where the subscribers can switch an active data call session between fixed wireless network (e.g., a wireless local area network (WLAN)) and mobile network (e.g., a cellular network) (See, e.g., Choudhury, discussed above). As discussed in Choudhury, Airvana and PicoChip above, the benefits of femtocells include (a) improved indoor coverage, (b) reduced capital and operational expenditures, (c) reduced bandwidth load, (d) reduced power requirement, (e) additional high-end revenue streams, (f) improved customer royalty, (g) increase in the average revenue per user, (h) compatibility with existing handsets, and no requirement of dual-mode terminals, (i) deployment in operator-owned spectrum, and (j) enhanced emergency services (since the femtocells will know their locations).
While the femtocell promises many benefits, the technology is still at its infancy, with many technical issues still to be solved. One problem impeding femtocells from practical deployment is radio interference management (i.e., interferences between a macro-cellular network (“macrocell”) and the femtocell, and between femtocells), which must be minimized. Moreover, there are still many open issues related to how to handle hand-offs between a macrocell and a femtocell, security aspects, scalability problems and access control. See, e.g., the discussions in Airvana.
The literature that addresses these personal base station problems is limited, and is typically only applicable to code division multiple access (CDMA) based technologies. However, next-generation wireless systems (e.g., Long Term Evolution (LTE) and IMT-Advanced systems) are likely to use a frequency division multiple access technology, such as orthogonal frequency division multiple-access (OFDMA) and single-carrier frequency division multiple access (SC-FDMA). Hence, the femtocells in future wireless networks are likely to use OFDMA or SC-FDMA technology, which has a different set of problems, as compared to CDMA networks. In particular, interference management and coexistence between the OFDMA-based (or SC-FDMA-based) macrocell network and the femtocell network are challenging issues that require careful design.
Further, a generic model and framework for a femtocell coexisting with a macrocell network is not available in the literature. The uplink (UL) capacity of a femtocell network that coexists with a macrocell network (i.e., a shared-spectrum network) is derived and analyzed in the article “Uplink Capacity and Interference Avoidance for Two-Tier Cellular Networks” (“Chandrasekhar”), by Vikram Chandrasekhar and Jeffrey G. Andrews, in Proc. IEEE Global Telecommunications Conference (GLOBECOM), pp. 3322-3326, November 2007. In a split spectrum network, the femtocell users and the macrocell users are assigned sub-channels that are orthogonal to each other. While such a division avoids interference between the macrocell and the different femtocells, the total number of users that can be supported is diminished, especially when a large number of femtocells are provided within a macrocell. For a shared spectrum network, a femtocell may utilize some sub-channels that are also utilized by the macrocell, so long as there is limited interference between the two networks. To improve the outage probability, Chandrasekhar proposes using interference avoidance methods. In particular, the macrocell and each femtocell may use time-hopping to decrease the interference. Further, a sectored antenna may be used to provide reception for both the macrocell and femtocell, so as to achieve better capacity. Through interference avoidance (time-hopped CDMA and sectorized antennas), analytical and simulation results show that a femtocell base station (BS) density which is up to seven times higher than without interference avoidance (e.g., relative to a split spectrum network with omnidirectional femtocell antennas) can be supported.
The article, entitled “Effects of User-Deployed, Co-Channel Femtocells on the Call Drop Probability in a Residential Scenario” (“Ho”), by Lester T. W. Ho and Holger Claussen, published in Proc. of IEEE Int. Symp. on Personal, Indoor and Mobile Radio Communications (PIMRC), pp. 1-5, September 2007, analyses femtocells and handover probabilities for different power configurations at a femtocell. Since manual cell planning used in macrocell networks is not practicable for femtocells (i.e., not economical), femtocells typically require auto-configuration capabilities, such as femtocell power and cell size auto-configuration. Using simulations, Ho shows that call-drop probabilities in a residential co-channel femtocell deployment can be significantly decreased through simple pilot power adaptation mechanisms.
The article “Performance of Macro- and Co-Channel Femtocells in a Hierarchical Cell Structure” (“Claussen”), by Holger Claussen, published in Proc. of IEEE Int. Symp. on Personal, Indoor and Mobile Radio Communications (PIMRC), pp. 1-5, September 2007, discloses a simple power control algorithm for pilots and data in a femtocell. Simulation results show that the interference with the macrocell network can be minimized using such a power control algorithm.
Detection of subcarriers that are already being used is a critical component in an OFDMA-based femtocell. By sensing the subcarriers that are being used by the macrocell network, a femtocell can avoid using these subcarriers. Note that these detected subcarriers may be used by users who are sufficiently far away from the femtocell to allow usage by the femtocell. A challenge in detecting such subcarriers is the lack of time or frequency synchronization between the signals arriving from different macrocell mobile stations (mMSs) at the femtocell BS (fBS). Therefore, the fBS must detect the used subcarriers by spectrum sensing, without time or frequency synchronization. Spectrum sensing is discussed in the following references:                (a). Sheng-Yuan Tu, Kwang-Cheng Chen, and Ramjee Prasad, “Spectrum Sensing of OFDMA Systems for Cognitive Radios” (“Tu”), in Proc. IEEE Int. Symp.        
On Personal, Indoor, and Mobile Radio Communications (PIMRC), 2007.                (b). Nilesh Khambekar, Liang Dong, and VipinChaudhary, “Utilizing OFDM Guard Interval for Spectrum Sensing” (“Khambekar”), in Proc. IEEE Int. Symp. On Personal, Indoor, and Mobile Radio Communications (PIMRC), 2007.        (c) Ghurumuruhan Ganesan and Ye (Geoffrey) Li, “Cooperative Spectrum Sensing in Cognitive Radio, Part I: Two User Networks” (“Ganesan I”), IEEE Trans.        
Wireless Communications, vol. 6, no. 6, pp. 2204-2213, June 2007.                (d) Ghurumuruhan Ganesan and Ye (Geoffrey) Li, “Cooperative Spectrum Sensing in Cognitive Radio, Part II: Multiuser Networks” (“Ganesan II”), IEEE Trans. Wireless Communications, vol. 6, no. 6, pp. 2214-2222, June 2007.        (e) F. S. Chu and K. C. Chen, “Radio Resource Allocation in OFDMA Cognitive Radio Systems” (“Chu”), in Proc. IEEE Personal, Indoor and Mobile Radio Commun. (PIMRC), pp. 1-5, September 2007.        (f) T. H. Kim and T. J. Lee, “Spectrum Allocation Algorithms for Uplink Sub-carriers in OFDMA-Based Cognitive Radio Networks” (“Kim”), in Proc. IEEE Int. Conf. on Innovations in Information Technol., pp 51-54, November 2007.        
In Tu, the Lloyd-Max algorithm is used for channel identification in a cognitive radio system, and a two-dimensional resource allocation algorithm is disclosed. Khambekar discloses spectrum allocation algorithms for uplink subcarriers in OFDMA-based cognitive radios. In Khambekar, subcarriers that are detected unused by the primary network are assigned to a secondary network based on carrier to interference plus noise ratio (CINR) and throughput considerations. For subcarriers that are used by the primary network, the ones that yield the lowest interference to the primary network, or the ones that have the largest CINR are assigned to the secondary MS. Tu and Khambekar, however, do not address issues that are uniquely related to OFDMA systems and femtocells, such as considerations of time or frequency asynchronization, or utilization of scheduling information from the macrocell BS.
The cognitive radio algorithms which are applicable to femtocells do not take advantage of any collaboration between the primary system (i.e., the macrocells) and the secondary system (i.e., the femtocells), such as providing the frequency allocation maps from the primary system. Instead, the femtocells are required to avoid all the frequency bands that appear “occupied” regardless of the level of risk of posed to the primary users at their respective locations. Hence, very limited portions of the spectrum are available to secondary networks. Since such cognitive radio algorithms require perfect avoidance of co-channel interference, cancelling the co-channel interference caused by primary systems is not developed in the cognitive radio algorithm context.
Nylander, Linqvist and Vikberg disclose new development in femtocell systems in the following works:                (a) T. Nylander, J. Vikberg, P. M. Teder, “Access Control in Radio Access Network Having Pico Base Stations” (“Nylander”), U.S. Patent Application Publication, U.S. 2007/0183427, filed Oct. 3, 2006.        (b) Thomas Lars Erik Lindqvist, Tomas Nylander, and Jari Tapio Vikberg, “Dynamic Building of Monitored Set” (“Lindqvist”), U.S. Patent Application Publication U.S. 2007/0254620, filed Apr. 28, 2006.        (c) Jari Vikberg and T. Nylander, “Method and Apparatus for Remote Monitoring of Femto Radio Base Stations” (“Vikberg”), Application Number: WO2007/0139460, published Dec. 6, 2007.        
However, none of these disclosures addresses coexistence issues among femtocells and macrocells in an OFDMA-based system.